This invention is related to semiconductor devices. In particular, the invention is related to high voltage heterostructure field effect transistors (HFETs) that operate as power switches.
High voltage semiconductor switches are key components in electronic circuits for power conversion. Examples of these applications include power supplies for electronic equipment, drives for electric motors, and inverters for solar cells.
A power switch has an on state that allows the device to conduct current, and an off state that prevents the device from conducting current. When in the on state, a power switch may conduct tens or hundreds of amperes while the voltage across the switch is less than one volt. When in the off state, the power switch typically must withstand hundreds or thousands of volts while conducting substantially zero current. The voltage that the device can withstand in the off state while conducting no more than a given small value of current is sometimes referred to as the breakdown voltage.
It is usually desirable that the transition between the two states be as fast as possible because during the transition there may be relatively high current in the switch at the same time there is relatively high voltage across the switch. The presence of relatively high current and high voltage at the same time represents a loss of energy that at best is undesirable and at worst could destroy the switch.
High voltage HFETs are attractive for use as power switches primarily because they can change states substantially faster than other semiconductor switches that conduct the same currents at similar voltages. The materials used in the construction of HFETs also allow them to operate at higher temperatures than transistors that use traditional silicon-based technology.
A major problem with the fabrication of HFETs for power switches is that existing techniques produce devices with breakdown voltages that are substantially below the values that are theoretically possible. In addition, it is difficult to predict how the application of known techniques such as the use of field plates influences the breakdown voltage. As such, the design of devices for specific characteristics is difficult and time-consuming, requiring the application of cut-and-try methods to fabricate and test hardware rather than the use of computer modeling.
A solution is required that will allow power HFETs to be designed in less time for desired performance with predictable breakdown voltage.